Acoustic Drop Calibration

My front tires held position beside the resonant iron post. Ahead, halfway toward the outer yellow border, rested a dry acorn shell, concave and clean. Vertical descent corridors require periodic echo tests. Centering my front bumper against its curved lip, I applied low forward power in three deliberate pulses. The shell slid over the gritty concrete, caught the edge, and fell. It tumbled through the open drop, striking the next tread below with a sharp, hollow crack. The return sound carried clearly up the shaft. The lower zone remains clear.